Heat pumps have become increasing popular because of the energy efficiency in transferring rather than creating heat. A heat pump typically includes a compressor which circulates refrigerant through a first heat exchanger or condenser, through an expansion valve or opening, through a second heat exchanger or evaporator, and into a refrigerant storage device or accumulator. Vapor is withdrawn from the storage device for recirculation by the compressor. A heat pump can commonly be operated in either a heating or cooling mode by selective activation of a reversing valve.
In preferred operation, the refrigerant vapor is fully condensed to liquid at the exit of the condenser. In addition, the refrigerant is preferably fully vaporized at the exit of the evaporator as disclosed in U.S. Pat. Nos. 4,573,327 and 4,665,716, both to Cochran and assigned to the assignee of the present invention. These patents further describe significant advances in the area of control of refrigerant flow using a charge control device in place of a conventional accumulator, and a float-type liquid control valve connected between the first and second heat exchangers to thereby enhance control of refrigerant flow and increase energy efficiency.
Air source heat pumps which exchange heat with ambient air have been most common because of their generally low initial cost. Another type of heat pump is the ground-coupled heat pump which transfers heat with the ground through a heat exchanger commonly called an earth loop or earth tap. A ground-coupled heat pump is typically more efficient than an air source heat pump because the earth temperature may be more stable than ambient air.
Among the ground-coupled heat pumps are the direct expansion and closed loop type. The closed loop heat pump typically includes an intermediate fluid, such as an antifreeze solution, which is circulated between one or more buried conduits and a heat exchanger as disclosed in U.S. Pat. No. 4,325,228. In other words, an extra stage of heat exchange is required in the closed loop heat pump.
The direct expansion heat pump circulates refrigerant directly through an earth tap heat exchanger and may be more efficient than a closed loop heat pump. The term direct expansion derives from the evaporation and expansion of the liquid refrigerant to vapor in the buried heat exchanger. In addition, the direct expansion heat pump does not require pumping of an intermediate fluid as does the closed loop heat pump. A ground-coupled direct expansion heat pump may require a relatively large amount of refrigerant compared with an air-source heat pump or a closed loop heat pump.
Direct expansion heat pumps have used a U-shaped earth tap heat exchanger, including two parallel conduits joined in fluid communication at their lower ends, and wherein one conduit carries liquid and the other vapor. Coaxial or concentric tubes for liquid and vapor refrigerant have also been used as disclosed, for example, in German Pat. No. 3,203,526A. Typically a layer of insulation is provided between the coaxial tubes.
Conventional earth taps typically include tubing comprising copper or other metallic materials to provide the vapor and liquid carrying passageways. Unfortunately, copper, for example, may form an oxide layer in most soil conditions. Such an oxide layer may decrease the thermal transfer capacity of the tube. In addition, advanced corrosion may cause an opening in the tube thereby leaking refrigerant into the soil. Accordingly, copper tubing for an earth tap typically requires a relatively large wall thickness beyond what is needed for the typical operating pressures. In addition, the integrity of an earth tap heat exchanger may be checked only by cumbersome pressure testing procedure after installation to thereby ensure that no damage has occurred to the tubes during installation.
Since many earth taps are typically connected to a common manifold, each must be individually checked by pressurizing the tube and detecting a loss of pressure to identify the leaking earth tap. The leaking earth tap heat exchanger may then be repaired or replaced. In other words, checking the integrity of a plurality of earth taps may be time consuming and difficult. Accordingly, the cost and complexity of installing and maintaining an earth tap heat exchanger may be relatively high.